1. Field of the Invention
The present invention relates to an electrostatic capacitance type transducer used for a pressure sensor, an acceleration sensor, and so on.
2. Description of the Related Art
Conventionally, an electrostatic capacitance type transducer has been used for measuring instruments such as a pressure sensor or an acceleration sensor.
The electrostatic capacitance type transducer has a structure to have a fixed electrode and a movable electrode confronting each other in a manner that a substrate having the film-like fixed electrode is placed in front of the movable electrode through a space so that a displacement of the movable electrode to the substrate is detected as a change in electrostatic capacitance between the movable electrode and the fixed electrode.
For instance, in a electrostatic type pressure sensor, a diaphragm as a movable electrode is formed of a conductive silicon. When a fluid pressure is measured by the electrostatic capacitance type pressure sensor, the fluid is introduced on the surface opposite to the substrate surface of the diaphragm, and the pressure of the fluid is converted into an electric signal by detecting the displacement of the diaphragm resulting from the pressure of the fluid as the change in the electrostatic capacitance.
The fixed electrode of such electrostatic capacitance type transducer is derived from the detecting face through a through hole and the like which is formed on the substrate, and is electrically connected to a signal processing circuit by connecting lead wire to the derived portion.
Since an aluminum wire or a gold wire is usually used for the lead wire, materials for the fixed electrode are limited to those which are capable of bonding to these wires. Among them, aluminum is widely used because it is moderate in price, superior in adherence with glass used for the substrate, and easy to carry out a photo-lithographical processing for forming patterns.
There is a disadvantage that, however, when the fixed electrode is formed with an aluminum film, it is apt to produce a protrusion called a "hillock" on the surface of the fixed electrode during measurement under high temperature or during a heating process in the production, and the distance between the fixed electrode and the movable electrode is changed by the hillock, thus disturbing a precise detection of the displacement of the movable electrode.
Particularly, since the distance between the fixed electrode and the movable electrode is decreased as the transducer becomes smaller in size, the effect of the hillock can not be ignored and there arises a possibility of detection failure due to contact of the grown up hillock with the movable electrode.
Aluminum is apt to be more subject to corrosion in a severe environment, especially in a corrosive environment such as existence of vapor condensation, acidic and alkaline substances in the atmosphere and so forth. So when aluminum is used for the fixed electrode, there arises a disadvantage that the life of the transducer is relatively shorter than expected.
Moreover changes in shape of the fixed electrode surface due to corrosion, affects the space between the movable electrode and the fixed electrode, which causes deterioration in the detection accuracy.
It is an object of the present invention to provide an electrostatic capacitance type transducer which ensures an excellent bonding property, high detection accuracy and an improvement of the corrosion resistance.